Bob Barr, former GOP congressman from Georgia, is an all-but-announced presidential candidate – as a Libertarian.

The possibility of a run by Barr has sent shudders through the mainstream of the Republican party.

Barr, who will probably not declare his intentions for several days, has already been labeled a “spoiler.”

In an interview with the Inquirer, Barr dismissed those accusations as whining.

“The notion that Republicans see a third-party candidate as spoiling their chances simply illustrates the arrogance of the two-party system,” Barr said.

Republicans may have good cause to worry.

A run by Barr could be to John McCain “what Ralph Nader was to Al Gore – ruinous,” wrote George Will in Newsweek. Some party experts believe Barr could siphon off essential conservative votes from Sen. John McCain, about whom many rightward voters have been less than enthusiastic.

Then, here’s him at CPAC 2008 introducing Ron Paul, and it’s a love letter to the congressman.

Think Ron Paul will tell his supporters to get behind Bob Barr? Think those Paulites who might be considering voting for McCain could swing to Barr as a real alternative? Quite possibly